Carbon fibers have been applied for sporting goods and aerospace materials because of their excellent specific strength and specific modulus, and are being used in wider ranges in these fields.
On the other hand, carbon fibers are also used for forming energy related apparatuses such as CNG tanks, fly wheels, wind mills and turbine blades, as materials for reinforcing structural members of roads, bridge piers, etc., and also for forming or reinforcing architectural members such as timber and curtain walls.
Since that carbon fibers are being applied in wider fields, they are demanded to have higher tensile strength when expressed as a resin impregnated strand than before. Further expanding applicable fields, the carbon fibers are demanded to be produced at lower cost.
The conventional techniques for improving tensile strength of carbon fibers as a resin impregnated strand have been concerned with decrease of macro-defects, for example, for decreasing impurities existing inside single filaments constituting the carbon fibers, or for inhibiting the production of macro-voids formed inside the single filaments, and for reducing defects generated on the surfaces of the single filaments.
To decrease the inner impurities and macro-voids of single filaments, techniques to intensify the filtration of monomer or polymer dope are proposed in Japanese Patent Laid-Open (Kokai) No. 59-88924 and Japanese Patent Publication (Kokoku) No. 4-12882. Furthermore, techniques to inhibit the production of surface defects by controlling the shape of fiber guides used in the production process of precursor fibers or controlling the tension of fibers in contact with a guide are proposed in Japanese Patent Publication (Kokoku) No. 3-41561.
Although they were effective in improving strength in the past, when the tensile strength level of carbon fibers as a resin impregnated strand was low, the techniques have already achieved their intended effects of strength improvement, as impurities and macro-voids have been almost perfectly removed. In other words, these techniques cannot be expected to improve the strength further.
Furthermore, when precursor fibers are stabilized and carbonized at a high temperature to produce carbon fibers, coalescence between single filaments is likely to occur, and the coalescence between single filaments and marks that remain after their separation cause surface defects, and lower fiber strength.
To inhibit coalescence between single filaments, techniques for impregnating precursor fibers with fine particles of graphite in the production process of precursor fibers are proposed in Japanese Patent Laid-Open (Kokai) No. 49-102930 and Japanese Patent Publication (Kokoku) No. 6-37724, and a technique for impregnating precursor fibers with fine particles of potassium permanganate is proposed in Japanese Patent Publication (Kokoku) No. 52-39455.
The addition of these fine particles was effective in improving strength in the past when the coalescence between filaments occurred frequently and the tensile strength of carbon fibers as a resin impregnated strand was at a low level. However, today when the coalescence between filaments has been decreased to improve the strength level due to the application of the above techniques, these hard inorganic fine particles impregnated onto soft swelling fibers during production cause surface defects and lower the tensile strength of the carbon fibers when assembled as a resin impregnated strand.
Furthermore, to inhibit coalescence between single filaments, techniques are proposed to improve process oil as applied to precursor fibers. Techniques for applying silicone oils, which are excellent in lubricity and smoothness, instead of conventional non-silicone oils made from higher alcohols are proposed in Japanese Patent Publication (Kokoku) Nos. 60-18334 and 53-10175 and Japanese Patent Laid-Open (Kokai) Nos. 60-99011 and 58-214517.
Moreover, techniques for improving heat resistance of silicone oils are proposed in Japanese Patent Publication (Kokoku) Nos. 4-33862 and 58-5287, and Japanese Patent Laid-Open (Kokai) No. 60-146076. Particularly epoxy-modified silicone oils are proposed in Japanese Patent Publication (Kokoku) Nos. 4-29766 and 60-18334. The use of a mixture of amino-modified silicone and epoxy-modified silicone is proposed in Japanese Patent Publication (Kokoku) Nos. 4-33892 and 5-83642. The use of a mixture of an amino-modified silicone, epoxy-modified silicone and alkyleneoxide-modified silicone in combination is proposed in Japanese Patent Publication (Kokoku) No. 3-40152. However, even if these oils are applied, the coalescence between single filaments was not perfectly inhibited, in other words effect of inhibiting the coalescence between single filaments was not sufficient.
On the other hand, if these oils are improved in heat resistance, the deposition of oil gels (hereinafter called gum-ups) on the heating rollers, etc. existing downstream of the oiling process, increases problem s greatly in achieving stable production. Therefore, the equipment has to be stopped very frequently to remove the gum, or expensive gum removers must be installed which cause increased production cost.
Techniques to remove the surface defects generated in the precursor fiber production process, carbonization process or any subsequent processes are proposed. Techniques for heating carbon fibers in a dense inorganic acid are proposed in Japanese Patent Laid-Open (Kokai) No. 54-59497 and Japanese Patent Publication (Kokoku) No. 52-35796, and a technique for electrolyzing in inorganic acid at high temperature is proposed in Japanese Patent Publication (Kokoku) No. 5-4463. These techniques remove the generated surface defects by etching.
However, these techniques require inerting treatment of surface chemical functions excessively produced as a result of the etching treatment, to improve the strength of the composite material produced with these carbon fibers. The equipment, therefore, becomes complicated and it provides another cause for increase of production cost.
In addition to the macro-defects mentioned above, the strength is also affected by presence of micro-voids or micro-defects. Techniques are proposed to inhibit their generation. Techniques to densify precursor fibers for inhibiting the their generation are proposed. A technique to densify undrawn fibers by optimizing the conditions of the coagulating bath is disclosed in Japanese Patent Laid-Open (Kokai) No. 59-82420, and a technique to densify drawn fibers by keeping the drawing temperature in a bath as high as possible is disclosed in Japanese Patent Publication (Kokoku) No. 615722. However, since the techniques for achieving densification tend to lower oxygen permeability into the fibers in a stabilization process, the improvement in tensile strength expressed as a resin impregnated strand of the obtained carbon fibers, tends to be depreciated.
Therefore, the tensile strength of carbon fibers as a resin impregnated strand can be improved by these techniques only when precursor fibers are 0.8 denier or less in fineness of each single filament, or only when the carbon fibers are 6 .mu.m or less in the diameter of a single filament. For carbon fibers thicker than 6 .mu.m in diameter of a single filament, the improvement of tensile strength as a resin impregnated strand with these techniques is hard to obtain.
As for the polymer composition used to form precursor fibers, the use of any copolymerizable vinyl compound with acrylonitrile is proposed in Japanese Patent Laid-Open (Kokai) No. 59-82420, and copolymerization of .alpha.-chloroacrylonitrile, which is effective in lowering stabilization temperature, is proposed in Japanese Patent Publication (Kokoku) No. 6-27368. However, these proposals do not clarify the effect of improving strength.
Furthermore a technique designed to make the difference in oxygen content between the inner and outer layers of a stabilized single filament small, by copolymerizing an acrylate or methacrylate with acrylonitrile, is proposed in Japanese Patent Laid-Open (Kokai) No. 2-84505. However, the obtained precursor fibers are low in density and inhibition of the coalescence between single filaments is also insufficient. As a result, the tensile strength of carbon fibers as a resin impregnated strand, is as low as 5.1 GPa or less.
Precursor fibers made of polymer consisting of three or more components are proposed in Japanese Patent Publication (Kokoku) No. 6-15722. One of the components is specified as a stabilization accelerator which can be selected from acrylic acid, methacrylic acid, itaconic acid, their alkali metal salts and ammonium salts, and hydroxy esters of acrylic acid. Another component is specified as a spinning and drawing promoter which can be selected from lower alkyl esters of acrylic acid and methacrylic acid, allylsulfonic acid, methallylsulfonic acid, styrenesulfonic acid, their alkali metal salts, vinyl acetate and vinyl chloride. However, the effect in improving tensile strength as a resin impregnated strand by these components is not stated.
A technique to densify the structure of each single filament by making the temperature increase rate small or raising the tension of the fibers in the carbonization process is proposed in Japanese Laid-Open (Kokai) No. 62-110924. However, lowering the temperature increase rate means lowering carbonization speed and a larger apparatus, hence raising production cost. Raising the tension means lowering mechanical properties due to increase of fuzz in the fibers. Therefore, these techniques are limited in improving tensile strength.
Techniques to add fine particles of different compounds inside carbon fibers are proposed in Japanese Patent Publication (Kokoku) No. 61-58404 and Japanese Patent Laid-Open (Kokai) No. 2-251615 and 4-272236, and a technique to mix any of various resins with a polyacrylonitrile based polymer is proposed in Japanese Patent Laid-Open No. 5-195324. A technique in which atoms or molecules solid or gaseous at room temperature are ionized in vacuum and accelerated by an electric field, to be injected into the surface layer of each carbon fiber is proposed in Japanese Patent Laid-Open (Kokai) No. 3-18051.
However, in the case of carbon fibers containing fine particles, fine particles exist generally in each single filament and act as impurities to cut the single filaments in precursor production process and carbonization process, generating much fuzz. Therefore, these techniques lower the productivity, tensile strength and other mechanical properties of the carbon fibers.
A technique to mix fine particles containing a metal element, with the fibers faces a problem that compressive strength of the obtained carbon fibers is adversely affected, since catalytic graphitization generates larger graphite crystallites. Even if a polymer is mixed with resin, instead of the fine particles, it is difficult to obtain carbon fibers with a homogeneous structure, and as a result the tensile strength as a resin impregnated strand is lowered.
On the other hand, techniques proposed for improving productivity include a technique to raise the traveling speed of the fibers in the precursor production process or carbonization process, and a technique to increase number of single filaments per carbon fiber bundle. Although these techniques are effective in improving the productivity, they lower the tensile strength of the obtained carbon fibers (as a resin impregnated strand) at the present level of the techniques.
If the diameter (fineness) of single filaments constituting carbon fibers is increased, the tensile strength of the carbon fibers (as a resin impregnated strand) is greatly lowered disadvantageously at present level of techniques, although the productivity can be improved.
Japanese Patent Publication (Kokoku) No. 7-37685 proposes carbon fibers with a tensile strength of 6.5 GPa or more as a resin impregnated strand, but the diameter of single filaments disclosed is as small as 5.5 .mu.m or less, and carbon fibers with high tensile strength (as a resin impregnated strand) consisting of single filaments with a diameter larger than 6 .mu.m excellent are not disclosed.
In addition, since the technique must undergo a complicated process of electrolyzing in a high temperature electrolyte containing nitrate ions as an essential component, and subsequently heating in an inert atmosphere for adjusting surface chemical functions, the rise of production cost cannot be avoided. Though the carbon fibers obtained according to this technique are as thin as 5.5 .mu.m or less in single filament diameter, the tensile elongation of the carbon fibers as a resin impregnated strand is as low as 2.06% at the highest.
This suggests that if the single filament diameter is smaller, the modulus distribution in each single filament of carbon fibers becomes smaller, to raise the strength of carbon fibers, but at the same time, to raise the Young's modulus of the carbon fibers. So, even if the single filament diameter is smaller than 6 .mu.m, it is impossible to improve the tensile elongation of the carbon fibers as a resin impregnated strand to a value higher than 2.5%.
The technique to improve the tensile strength of carbon fibers as a resin impregnated strand by decreasing the fineness of single filaments has a limit, since single filaments javing a fineness of less than 0.5 denier are damaged remarkably in the production process of precursor fibers.